One type of a DC motor uses a double-layer structure armature coil that includes an inner layer coil and an outer layer coil. An armature core includes teeth, each having a distal end that is bifurcated into two branching teeth. The inner layer coil is wound around a basal end of each tooth. The outer layer coil is wound around the branching teeth of adjacent ones of the teeth.
In such a DC motor, the double-layer structure armature coil obtains sufficient torque and reduces vibration without enlarging the motor. Japanese Laid-Open Patent Publication No. 2011 55655 discloses a DC motor that includes an armature coil having a double-layer structure. In the DC motor disclosed in the above publication, due to the double-layer structure of the armature coil, the inductance of the coil is reduced to one-fourth when the number of turns is the same as that of a concentrated winding armature coil having a single-layer structure.
However, the inductance of the inner layer coil is larger than that of the outer layer coil. Thus, larger sparks are generated between a brush and a rectifier segment when the brush is separated from the inner layer coil through the rectifier than when the brush is separated from the outer layer coil through the rectifier.
This increases the abrasion caused by the sparks at the connected segment when the brush is separated from the inner layer coil. Thus, steps are formed in a circumferential direction in the rectifier. As a result, the life of the brush is shortened, and electromagnetic noise is easily generated.